In electrophotography an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image), is formed on a surface of an electrophotographic element comprising at least an insulative photoconductive layer and an electrically conductive substrate. The electrostatic latent image is usually formed by imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
In latent image formation the imagewise discharge is brought about by the radiation-induced creation of electron/hole pairs, which are generated by a material (often referred to as a charge-generation material) in the electrophotographic element in response to exposure to the imagewise actinic radiation. Depending upon the polarity of the initially uniform electrostatic potential and the type of materials included in the electrophotographic element, either the holes or the electrons that have been generated migrate toward the charged surface of the element in the exposed areas and thereby cause the imagewise discharge of the initial potential. What remains is a non-uniform potential constituting the electrostatic latent image.
Many electrophotographic elements currently in use are designed to be initially charged with a negative polarity. Such elements contain material which facilitates the migration of positive holes toward the negatively charged surface in imagewise exposed areas in order to cause imagewise discharge. Such material is often referred to as a hole-transport agent. In elements of that type, positively charged toner material is then used to develop the remaining imagewise unexposed portions of the negative polarity potential, i.e., the latent image, into a toner image. Because of the wide use of negatively charging elements, considerable numbers and types of positively charging toners have been fashioned and are available for use in electrographic developers.
However, for some applications of electrophotography it is more desirable to be able to develop the surface areas of the element that have been imagewise exposed to actinic radiation, rather than those that remain imagewise unexposed. For example, in laser printing of alphanumeric characters it is more desirable to be able to expose the relatively small percentage of surface area that will actually be developed to form visible alphanumeric toner images, rather than waste energy exposing the relatively large percentage of surface area that will constitute undeveloped background portions of the final image. In order to accomplish this while still employing widely available high quality positively charging toners, it is necessary to use an electrophotographic element that is designed to be positively charged. Positive toner can then be used to develop the exposed surface areas, which will have, after exposure and discharge, relatively negative electrostatic potential compared to the unexposed areas, where the initial positive potential will remain. An electrophotographic element designed to be initially positively charged preferably contains an adequate electron-transport agent, that is, a material which facilitates the migration of photogenerated electrons toward the positively charged insulative element surface.
Electrophotographic elements include both those commonly referred to as single layer or single-active-layer elements and those commonly referred to as multiactive, multilayer, or multi-active-layer elements.
Single-active-layer elements are so named because they contain only one layer that is active both to generate and to transport charges in response to exposure to actinic radiation. Such elements typically comprise at least an electrically conductive layer in electrical contact with a photoconductive layer. In single-active-layer elements, the photoconductive layer contains a charge-generation material to generate electron/hole pairs in response to actinic radiation and an electron-transport material, which comprises one or more of chemical compounds capable of accepting electrons generated by the charge-generation material and transporting them through the layer to effect discharge of the initially uniform electrostatic potential. The photoconductive layer is electrically insulative except when exposed to actinic radiation, and it sometimes contains an electrically insulative polymeric film-forming binder, which may itself be the charge-generating material, or it may be an additional material that is not charge-generating. In either case, the electron-transport agent is dissolved or dispersed as uniformly as possible in the binder film.
Multiactive elements are so named because they contain at least two active layers, at least one of which is capable of generating charge, i.e., electron/hole pairs, in response to exposure to actinic radiation and is therefore referred to as a charge-generation layer (CGL), and at least one of which is capable of accepting and transporting charges generated by the charge-generation layer and is therefore referred to as a charge-transport layer (CTL). Such elements typically comprise at least an electrically conductive layer, a CGL, and a CTL. Either the CGL or the CTL is in electrical contact with both the electrically conductive layer and the remaining CTL or CGL. The CGL contains at least a charge-generation material; the CTL contains at least a charge-transport agent; and either or both layers can contain an electrically insulative film-forming polymeric binder.
In solvent-coating a photoconductive layer of a single-active-layer element or a CGL and/or CTL of a multiactive element of the invention, a film-forming polymeric binder can be employed. If it is electrically insulating, the binder can help provide the element with electrically insulating characteristics. It also is useful for coating the layer and for adhering the layer to an adjacent layer. When it is a top layer, the polymeric binder provides a smooth, easy to clean, wear-resistant surface.
Binder polymers should provide little or no interference with the generation or transport of charges in the layer. Examples of binder polymers which are especially useful include bisphenol A polycarbonates and polyesters such as poly[(4,4'-norbornylidene) diphenylene terephthalate-co-azelate].
Electron mobility, a measure of the rate at which electrons migrate through the CTL, is defined as their velocity divided by the strength of the electric field. In general, the greater the electron mobility, the faster the electrophotographic process can be carried out.
Electrophotographic elements whose CTLs contain conventional binder polymers and anthraquinone bis-cyanoimine compounds as electron-transport agents exhibit good sensitometric properties; however electron mobility in such materials is low. A new type of binder polymer which enhances electron mobility without adversely affecting the above mentioned properties would be highly desirable. A binder polymer which can additionally function to transport electrons in a CTL containing no monomeric electron-transport agent would also be very useful. The present invention achieves these beneficial objectives.
Diimide compounds, both monomeric and polymeric, that contain aromatic tetracarbonyl groups are well known in the art and find use in various applications, including electrophotography. U.S. Pat. No. 4,992,349, for example, discloses monomeric naphthalenetetracarboxylic diimides as electron-transport agents in charge transport layers of electrophotographic elements. U.S. Pat. No. 4,668,600 discloses perylenetetracarboxylic diimides as n-type conducting pigments in a photoconductive layer of an electrophotographic recording material. U.S. Pat. No. 4,971,873 also discloses an electrophotographic element containing perylenetetracarboxylic diimide pigment in the charge generation layer and a polymeric binder that is a benzene tetracarboxylic polyarylimide that contains an indane group.
A method of preparing polyester-imides by reaction of tetracarboxylic acid dianhydrides with hydroxyalkyldicarboxylic acid amides is disclosed in U.S. Pat. No. 4,687,834. Japanese Patent Application J6 3110219 discloses a process of polycondensing perylenetetracarboxylic dianhydrides with aromatic diamines. Polymeric perylenetetracarboxylic diarylimides are disclosed as dyestuffs in DE 3,814,647.